Cells and batteries based on lithium metal come in many different forms and are used to power a wide range of electronic devices and instruments. Their distinguishing feature is the use of lithium metal (or a lithium metal alloy) as the anode. Like other electrochemical cells they require a counter electrode or cathode and a lithium-ion conducting electrolyte joining the two electrodes. The anode and counter electrode must also be electronically separated. Since most lithium cell electrolytes are liquid, this is usually accomplished through the use of an insulating porous film impregnated with the electrolyte, sandwiched between the two electrodes.
A wide range of cathode materials have been used or proposed for use in cells of this configuration. Some desirable properties for cathode materials include high voltage, high energy density, compatibility with the electrolyte, and high conductivity when utilized in a cell.
Some examples of commercial lithium metal batteries with liquid electrolytes and porous separators include lithium/carbon monofluoride, lithium/iron disulfide, lithium/manganese dioxide and lithium/silver vanadium oxide. These cells operate well at room temperature with relatively high power and energy density. Other cathode materials have been proposed. Of particular relevance to this invention, a liquid electrolyte cell with a porous polymer separator in this classic configuration has been reported in the journal Dopovidi Natsional'noi Akademii Nauk Ukraini, 1997, 3, p. 154 utilizing a lithium anode and a cathode containing black phosphorus.
A number of lithium cells have been developed that utilize a solid electrolyte, which can function both as the separator and the electrolyte. Solid electrolytes for lithium cells must be both lithium ion conducting and electronically insulating. There have been a wide range of solid state materials identified over the years as suitable for use as electrolytes for lithium cells including lithium iodide, lithium phosphide, LiPON and LiSiCON. Most of these materials are glassy in nature and their lithium ion conductivities can vary greatly based on their form, specific composition and temperature of operation. Other solid electrolytes include polyethylene oxide/lithium salt composites and solid polymer gel electrolytes. The solid electrolyte material can be applied as a thin layer between the anode and cathode and if necessary mixed with the active anode and cathode materials within the electrodes. Some examples of cathode materials used or proposed for use in such cells include sulfur, iodine, and lithium transition metal oxides such lithium vanadium oxide, lithium iron phosphate, and lithium cobalt oxide. Cells made with solid electrolytes often have advantages in terms of storage stability, energy density, ease of manufacture, wide temperatures of operation and resistance to shock and vibration.
Within this class of solid-state cells and batteries, the lithium/iodine cell is unique in that the reaction between the anode and cathode itself generates the solid electrolyte phase, lithium iodide (LiI). Since the solid electrolyte phase is generated in-situ, there is no need to apply a solid electrolyte film to either the anode or cathode prior to cell assembly leading to advantages in energy density, manufacturability and life. Such a process also prevents shorting of the cell via self-healing, making them highly reliable and safe. The electrolyte propagates through the active materials as the cell is discharged and more reaction product or electrolyte is formed. Thus, as the anode and cathode are consumed by the discharge reaction, the electrolyte formed occupies the space, keeping the two electrodes in ionic contact. The theoretical energy density of the lithium/iodine couple is ˜1.9 Wh/cm3 with current practical values approaching 1 Wh/cm3. Such cells are described in U.S. Pat. Nos. 3,660,163; 3,674,562; 4,148,975 and 4,952,469.
Due to their reliability and long life, these cells are widely used to power cardiac pacemakers and have been used for solid-state memory power, digital watches and sensors and monitoring equipment. While the voltage of operation is high (˜2.8 V) lithium/iodine cells operate at very low power and high impedance, due to the low ionic conductivity of the electrolyte (˜10−7/Ohm-cm) and the low electronic conductivity of the iodine. Methods of reducing the high impedance of the iodine cathode have been developed and most commercial cells currently utilize a pyridine-containing polymer (P2VP) as a depolarizer to increase the conductivity of the solid iodine phase. Such approaches lead to a decrease in the percentage of active material in the cell and thus a decrease in overall energy density. However, the advantages of the self-forming electrolyte and relative high capacity have led to widespread use in commercial devices.
Accordingly, a new solid-state cell with a self-forming electrolyte that exhibits greater energy density and higher power would be highly desirable.